Communication system and method for mitigating interference in hierarchical cell structure

ABSTRACT

A method for mitigating interference at a small base station in a hierarchical cell structure is provided. In the method, the small base station checks a sub-frame allocated to macro user equipment by using a downlink signal received from a macro base station when an interference candidate user equipment list is received. The interference candidate user equipment list contains information on the macro user equipment adjacent to a femto cell controlled by the small base station. Then the small base station allocates other sub-frame to femto user equipment located in the femto cell by avoiding the sub-frame allocated to the macro user equipment.

PRIORITY

This application is a continuation of prior application Ser. No.12/752,584, filed on Apr. 1, 2010, which claimed the benefit under 35U.S.C. §119(a) of a Korean patent application filed in the KoreanIntellectual Property Office on Apr. 1, 2009 and assigned Serial No.10-2009-0027957, the entire disclosures of which is hereby incorporatedby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Aspects of the present invention relate to a method for mitigatinginterference between user equipment in a hierarchical cell structure andto a communication system executing the above method. More particularly,aspects of the present invention relate to a method for mitigatinginterference which may occur between user equipment located in a macrocell and a femto cell when the femto cell containing a small basestation is disposed within the macro cell controlled by a macro basestation, and also to such a macro base station and such a small basestation executing the above method.

2. Description of the Related Art

FIG. 1 is a schematic view illustrating a communication system with ahierarchical cell structure according to the related art.

Referring to FIG. 1, a communication system includes a macro basestation 100 as a normal cellular network, and a plurality of small basestations 200 a, 200 b, and 200 c. Normally, a small base station, suchas the plurality of small base stations 200 a, 200 b, or 200 c, isdesigned as an in-house base station for use in residential or smallbusiness environments, hence covers an extremely limited range, and isoften referred to as a femto cell or a pico cell. The small base station200 a, 200 b, or 200 c for controlling such small cells is oftenreferred to as a Home Node B (HNB). The cell coverage area of the smallbase station 200 a, 200 b or 200 c is much smaller than that of themacro base station 100.

Since the macro base station 100 covers a broader range than the smallbase station 200 a, 200 b, or 200 c, the macro base station 100 performsfrequency allocation using a Frequency Division Duplex (FDD) scheme. Thesmall base station 200 a, 200 b, or 200 c performs frequency allocationusing a Time Division Duplex (TDD) scheme. The FDD scheme supportstwo-way radio communication through two distinct frequency channels. TheTDD scheme uses a single frequency channel with time slots to transmitsignals in both the downstream and upstream directions.

FIG. 2 is a schematic view illustrating an allocation of frequency bandsin a communication system including a macro base station and a smallbase station according to the related art.

Referring to FIG. 2, the frequency bandwidth allocated to eachcommunication system may be 2 GHz or 2.5 GHz.

In FIG. 2, case (a) shows the allocation of frequency bands for themacro base station and the small base station on the assumption that thefrequency bandwidth is 2 GHz. In case (a), two frequency bands 1900˜1920MHz and 2010˜2025 MHz are allocated to TDD channels used in the smallbase station 200 a, 200 b, or 200 c. Other frequency bands 1920˜1980 MHzand 2110˜2170 MHz are allocated respectively to uplink (UL) and downlink(DL) channels of FDD used in the macro base station 100.

Case (b) in FIG. 2 shows the allocation of frequency bands for the macrobase station and the small base station on the assumption that thefrequency bandwidth is 2.5 GHz. In case (b), two frequency bands2500˜2570 MHz and 2620˜2690 MHz are allocated respectively to uplink(UL) and downlink (DL) channels of FDD used in the macro base station100. Another frequency band 2570˜2620 MHz is allocated to a TDD channelused in the small base station 200 a, 200 b or 200 c.

In frequency allocation environments as shown in FIG. 2, a frequencyband allocated to FDD channels used in the macro base station may beadjacent to another frequency band allocated to TDD channels used in thesmall base station. In this case, unfavorable interference may oftenoccur. The following are examples of the types and features of theinterference.

1) Macro Base Station 100 and Small Base Station 200 a, 200 b, or 200 c

Serious interference may occur where the macro base station 100 and acertain one of the small base stations 200 a, 200 b, and 200 c are closeto each other. In this case, interference may be mitigated by reducing atransmitting power of the small base station at issue and by increasinga physical distance from the macro base station.

2) Macro Base Station 100 and User Equipment Located in Femto CellControlled by Small Base Station 200 a, 200 b, or 200 c

Interference may occur when certain user equipment (namely, femto userequipment) located in a femto cell controlled by the small base station200 a, 200 b, or 200 c uses a high transmitting power near the macrobase station 100. In this case, interference may be mitigated bymaintaining a suitable transmitting power of the user equipment at issueand by regulating the coverage of a related small base station.

3) Small Base Station 200 a, 200 b, or 200 c and User Equipment Locatedin Macro Cell Controlled by Macro Base Station 100

Interference may occur when certain user equipment (namely, macro userequipment) located in a macro cell controlled by the macro base station100 uses a high transmitting power near the small base station 200 a,200 b, or 200 c. In this case, the handover for the user equipment atissue may be performed to one of the small base stations 200 a, 200 b,or 200 c. The loss of performance due to interference may not be heavysince there is a low probability that the macro user equipment willexist near the small base stations.

4) Macro User Equipment and Femto User Equipment

FIG. 3 is a schematic view illustrating an occurrence of interferencebetween user equipment according to the related art.

Referring to FIG. 3, interference may occur as indicated by a dashedarrow when certain macro user equipment 320 located in a macro cellcontrolled by the macro base station 100 is close to certain femto userequipment 340 located in a femto cell controlled by the small basestation 200. For example, interference may occur when the macro userequipment 320 uses a high power near the femto user equipment 340 inorder to transmit signals to the macro base station 100. This kind ofinterference may cause the heaviest loss of performance, so a moreefficient method for mitigating interference is required.

In order to mitigate interference between the macro user equipment 320and the femto user equipment 340, a transmitting power control method, ahandover method, and a dynamic channel allocation method may beconsidered.

The transmitting power control method increases a transmitting power ofthe small base station 200 in a case where the femto user equipment 340suffers interference from the macro user equipment 320. However, thismethod may instead increase interference in the uplink channel of themacro user equipment 320.

The handover method performs handover of the macro user equipment 320 tothe small base station 200 or of the femto user equipment 340 to themacro base station 100 when interference occurs. However, considering ahandover delay time, a closed access scenario of the small base station,a load balancing between cells, etc., this method may not be sufficientto mitigate interference. Although the handover may be somewhateffective in mitigating interference, this method needs an additionalway to mitigate interference.

The dynamic channel allocation method allocates different channels tothe macro user equipment 320 and the femto user equipment 340. Thismethod divides sub-frames, which are allocated on the basis of time,into first sub-frames used in the macro user equipment 320 and secondsub-frames used in the femto user equipment 340 in order to avoidinterference. However, this method may have difficulty in regulating theamount of frames used in the macro user equipment 320 and the femto userequipment 340 respectively.

SUMMARY OF THE INVENTION

An aspect of the present invention is to address at least theabove-mentioned problems and/or disadvantages and to provide at leastthe advantages described below. Accordingly, an aspect of the presentinvention is to provide a method for mitigating interference in ahierarchical cell structure and also to a communication system executingthe above method.

In accordance with an aspect of the present invention, a method formitigating interference at a small base station in a hierarchical cellstructure is provided. The method includes checking a sub-frameallocated to macro user equipment via a downlink signal received from amacro base station when an interference candidate user equipment list isreceived, the interference candidate user equipment list containinginformation on the macro user equipment adjacent to a femto cellcontrolled by the small base station, and allocating other sub-frames tofemto user equipment located in the femto cell by avoiding the sub-frameallocated to the macro user equipment.

In accordance with another aspect of the present invention, a method formitigating interference at a macro base station in a hierarchical cellstructure is provided. The method includes determining a ratio of uplinkto downlink in a small base station, detecting macro user equipmentadjacent to the small base station, and allocating a sub-framecorresponding to an uplink time slot of the small base station to themacro user equipment.

In accordance with still another aspect of the present invention, amethod for mitigating interference in a communication system with ahierarchical cell structure is provided. The method includes, at a macrobase station, detecting macro user equipment adjacent to a small basestation after time synchronization between the macro base station andthe small base station, at the macro base station, transmitting aninterference candidate user equipment list to the small base station,the interference candidate user equipment list containing the detectedmacro user equipment, at the small base station, detecting a sub-frameallocated to the macro user equipment contained in the interferencecandidate user equipment list, and at the small base station, allocatinga downlink sub-frame to femto user equipment by avoiding an uplinksub-frame allocated to the macro user equipment.

In accordance with yet another aspect of the present invention, acommunication system for mitigating interference is provided. The systemincludes a macro base station for determining the ratio of uplink todownlink in a small base station after time synchronization with thesmall base station, for detecting macro user equipment adjacent to thesmall base station, and for allocating a sub-frame corresponding touplink time slot of the small base station to the macro user equipment,and the small base station for receiving an interference candidate userequipment list from the macro base station, for detecting the sub-frameallocated to the macro user equipment, and for allocating a downlinksub-frame to femto user equipment by avoiding an uplink sub-frameallocated to the macro user equipment.

Other aspects, advantages, and salient features of the invention willbecome apparent to those skilled in the art from the following detaileddescription, which, taken in conjunction with the annexed drawings,discloses exemplary embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of certainexemplary embodiments of the present invention will be more apparentfrom the following detailed description taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a schematic view illustrating a communication system with ahierarchical cell structure according to the related art;

FIG. 2 is a schematic view illustrating an allocation of frequency bandsin a communication system including a macro base station and a smallbase station according to the related art;

FIG. 3 is a schematic view illustrating an occurrence of interferencebetween user equipments according to the related art;

FIG. 4 is a flow diagram illustrating a method for mitigatinginterference between communication systems according to an exemplaryembodiment of the present invention;

FIG. 5 is a flow diagram illustrating a method for mitigatinginterference at a macro base station according to an exemplaryembodiment of the present invention;

FIG. 6 is a flow diagram illustrating a method for performing avoidanceallocation to mitigate interference at a small base station according toan exemplary embodiment of the present invention; and

FIG. 7 is a flow diagram illustrating a method for calculating proximitybetween user equipments according to an exemplary embodiment of thepresent invention.

Throughout the drawings, it should be noted that like reference numbersare used to depict the same or similar elements, features andstructures.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The following description with reference to the accompanying drawings isprovided to assist in a comprehensive understanding of exemplaryembodiments of the invention as defined by the claims and theirequivalents. It includes various specific details to assist in thatunderstanding but these are to be regarded as merely exemplary.Accordingly, those of ordinary skill in the art will recognize variouschanges and modifications of the embodiments described herein can bemade without departing from the scope and spirit of the invention. Inaddition, descriptions of well-known functions and constructions areomitted for clarity and conciseness.

The terms and words used in the following description and claims are notlimited to the bibliographical meanings, but are merely used by theinventor to enable a clear and consistent understanding of theinvention. Accordingly, it should be apparent to those skilled in theart that the following description of exemplary embodiments of thepresent invention are provided for illustration purpose only and not forthe purpose of limiting the invention as defined by the appended claimsand their equivalents.

It is to be understood that the singular forms “a,” “an,” and “the”include plural referents unless the context clearly dictates otherwise.Thus, for example, reference to “a component surface” includes referenceto one or more of such surfaces.

As used herein, a macro base station refers to a base station offering acommunication service to a relatively broader range and is oftenreferred to as evolved Node B (eNB) or Node B. A macro cell refers to aservice area controlled by the macro base station. Macro user equipmentrefers to user equipment connected with the macro base station in orderto send and receive signals to and from the macro base station. A smallbase station refers to a base station located within the macro cell andoffering a communication service to a relatively smaller range. Thesmall base station is often referred to as a femto base station, a picobase station, or Home Node B (HNB). A femto cell refers to a servicearea controlled by the femto base station. Femto user equipment refersto user equipment connected with the small base station in order to sendand receive signals to and from the femto base station.

FIG. 4 is a flow diagram illustrating a method for mitigatinginterference between communication systems according to an exemplaryembodiment of the present invention.

Referring to FIG. 4, in step 410, both the macro base station 100 andthe small base station 200 perform a time synchronization process. Thesmall base station 200 receives a downlink signal from the macro basestation 100 and synchronizes the time with the macro base station 100using control information contained in the received signal. The controlinformation may include the allocation time of the sub-frames for macrouser equipment 320, the type of sub-frames allocated to the macro userequipment 320, and detailed information on signals transmitted throughsub-frames.

In step 420, the macro base station 100 determines the ratio of uplinkto downlink (U/D ratio) in the small base station 200 located in a macrocell. This U/D ratio may be determined through an exchange ofinformation between the small base station 200 and the macro basestation 100 when the small base station 200 is installed. The U/D ratiorefers to the distribution ratio of uplink sub-frames to downlinksub-frames allocated to femto user equipment 340 by the small basestation 200. Table 1 exemplarily shows the U/D ratio in the small basestation 200.

TABLE 1 UL-DL configura- Switch-point Sub-frame number tion periodicity0 1 2 3 4 5 6 7 8 9 0 5 ms D S U U U D S U U U 1 5 ms D S U U D D S U UD 2 5 ms D S U D D D S U D D 3 10 ms D S U U U D D D D D 4 10 ms D S U UD D D D D D 5 10 ms D S U D D D D D D D 6 5 ms D S U U U U U U U D

In step 430, the macro base station 100 detects the macro user equipment320 adjacent to the small base station 200. To detect the macro userequipment 320, the macro base station 100 receives a neighbor-cellmeasurement report from the macro user equipment 320 located in its owncell. The neighbor-cell measurement report contains the signal powerstrength of neighboring base stations measured by the macro userequipment 320. This report further contains specific identifiers ofrespective base stations. In an exemplary implementation, the macro basestation 100 may receive a plurality of neighbor cell measurement reportsfrom a plurality of respective macro user equipment 320.

In step 440, the macro base station 100 creates a list of interferencecandidate user equipment (UE) using information on the macro userequipment 320 adjacent to the small base station 200. The macro basestation 100 transmits the created list to the small base station 200.The macro base station 100 determines whether the signal power strengthbetween the small base station 200 and the macro user equipment 320contained in the neighbor-cell measurement report is greater than agiven threshold value. This threshold value may be smaller than anothervalue used for determining handover. The above threshold value may bedetermined through suitable experiments, which may consider severalconditions in which interference is likely to occur between the femtouser equipment 340 and the macro user equipment 320.

The macro base station 100 selects macro user equipment 320 with highersignal strength than a given threshold value and checks theidentifications of the selected macro user equipment 320. Thisidentification may include an identifier which has been already vestedin the respective macro user equipment 320 by the macro base station100. In addition, the macro base station 100 creates the interferencecandidate UE list using the identification of the selected macro userequipment 320. The macro base station 100 transmits the created list tothe small base station 200.

In step 450, the macro base station 100 allocates sub-framescorresponding to uplink time slots of the small base station 200 to themacro user equipment 320 adjacent to the small base station 200. Forexample, assume that the small base station 200 has the uplink/downlinkdistribution ratio as shown in Table 1. As shown in Table 1, sub-frames2, 3, and 7 in the small base station 200 have relatively higherprobabilities of uplink allocation. Therefore, when allocatingsub-frames to the macro user equipment 320, the macro base station 100allocates sub-frames 2, 3, and 7 in the order of adjacency to the smallbase station 200. Since the macro base station 100 allocates, in such amanner, uplink sub-frames of the small base station 200 to the macrouser equipment 320 adjacent to the small base station 200, it ispossible to mitigate interference which may occur between uplink of themacro user equipment 320 and downlink of the femto user equipment 340.

An exemplary method for performing avoidance allocation to mitigateinterference at the small base station 200 will be described withreference to FIG. 4. This method includes time synchronization (step410), detection of macro user equipment 320 adjacent to the small basestation 200 (step 430), transmission of an interference candidate UElist (step 440), detection of sub-frames allocated to the macro userequipment 320 (step 460), and allocation of sub-frames to the femto userequipment 340 while avoiding the detected sub-frames (step 470).

Steps 410 and 430 have been described above, so the repetition ofdescriptions will be avoided. As discussed previously, the macro basestation 100 obtains the interference candidate UE list through the step430. As also discussed above, in step 440, the macro base station 100transmits the interference candidate UE list to the small base station200. This list may be sent through a wired backhaul or a wirelessconnection such as Over-The-Air (OTA) between the macro base station 100and the small base station 200.

In step 460, the small base station 200 receives a downlink controlsignal from the macro base station 100 and detects sub-frames allocatedto the macro user equipment 320 contained in the interference candidateUE list. In step 470, the small base station 200 allocates sub-frames tothe femto user equipment 340 while avoiding the above-detectedsub-frames. A more detailed process of sub-frame allocation to the femtouser equipment 340 will be described later with reference to FIG. 6.

Hereinbefore, exemplary methods of sub-frame allocation executedrespectively at the macro base station 100 and the small base station200 in order to mitigate interference between the macro user equipment320 and the femto user equipment 340 have been described with referenceto FIG. 4. A method for mitigating interference at the macro basestation 100 will now be described in more detail.

FIG. 5 is a flow diagram illustrating a method for mitigatinginterference at a macro base station according to an exemplaryembodiment of the present invention.

Referring to FIG. 5, the macro base station 100 determines the ratio ofuplink to downlink (U/D ratio) in the small base station 200 usingdownlink signals received from the small base station 200 in step 510.

In step 520, the macro base station 100 detects the macro user equipment320 adjacent to the small base station 200. For such detection, themacro base station 100 may use a neighbor-cell measurement reportreceived from respective macro user equipment 320. Although not shown,the macro base station 100 creates the interference candidate UE listusing the identification of the macro user equipment 320 adjacent to thesmall base station 200. The macro base station 100 may transmit thecreated list to the small base station 200.

In step 530, the macro base station 100 allocates sub-framescorresponding to uplink time slots of the small base station 200 to themacro user equipment 320 contained in the interference candidate UElist.

An exemplary method for allocating sub-frames to the femto userequipments 340 in order to mitigate interference at the small basestation 200 will now be described in more detail.

FIG. 6 is a flow diagram illustrating a method for performing avoidanceallocation to mitigate interference at the small base station accordingto an exemplary embodiment of the present invention.

Referring to FIG. 6, in step 610, the small base station 200synchronizes the time with the macro base station 100 using downlinksignals of the macro base station 100. The small base station 200determines whether the interference candidate UE list is received fromthe macro base station in step 620. If it is determined in step 620 thatthe interference candidate UE list is received, the small base station200 proceeds to step 630 and initializes the value of proximity betweenthe femto user equipment 340 and interference candidate user equipmentcontained in the list. The initialization of the value of proximitybetween the femto user equipment 340 and interference candidate userequipment may be represented as an expression ‘I(A, B, n)=0’. Here, ‘A’refers to the interference candidate user equipment, ‘B’ refers to thefemto user equipment 340, and ‘n’ refers to the time index. Theinterference candidate user equipment refers to macro user equipmentlocated near a cell controlled by the small base station 200, among themacro user equipment 320 communicating with the macro base station 100.

In step 640, the small base station 200 checks uplink allocationinformation of the interference candidate user equipment. The small basestation 200 finds the interference candidate user equipment to whichuplink sub-frames are allocated. The interference candidate userequipment includes some of the macro user equipment 320 located in acell controlled by the macro base station 100. Therefore, uplinkallocation information of the interference candidate user equipment maybe checked through downlink signals transmitted from the macro basestation 100. The downlink signal from the macro base station 100 mayinclude a Physical Downlink Control Channel (PDCCH), which containsinformation on sub-frames allocated to each macro user equipment 320 andinformation on whether each allocated sub-frame is uplink or downlink.

In step 650, the small base station 200 determines success or failure inreceiving downlink at the femto user equipment. Depending on success orfailure, the small base station 200 updates the value of proximitybetween the femto user equipment 340 and the interference candidate userequipment in step 660. This updating step 660 is specifically shown inFIG. 7.

FIG. 7 is a flow diagram illustrating a method for calculating proximitybetween user equipment according to an exemplary embodiment of thepresent invention.

Referring to FIG. 7, in step 710, the small base station 200 determineswhether uplink sub-frames are simultaneously allocated to both the macrouser equipment 320 and the femto user equipment 340. If it is determinedin step 710 that uplink sub-frames are simultaneously allocated to boththe macro user equipment 320 and the femto user equipment 340, the smallbase station 200 proceeds to step 720 and further determines whether thefemto user equipment 340 fails to receive downlink.

If it is determined in step 720 that the femto user equipment 340 failsto receive downlink, the small base station 200 raises the value ofproximity between the femto user equipment 340 and the macro userequipment 320 in step 730. In this step, the small base station 200calculates the raised value of proximity by using Equation 1.I(A,B,n+1)=a*I(A,B,n)+(1−a)*T,0≦a≦1  Equation 1

Here, ‘I(A, B, n+1)’ refers to the raised value of proximity. Thereferences ‘A’, ‘B’ and ‘n’ refer to the macro user equipment 320, thefemto user equipment 340, and the time index, respectively. Thereferences ‘a’ and ‘T’ refer to a forgetting factor and a time function,respectively.

On the other hand, if it is determined in step 720 that the femto userequipment 340 succeeds in receiving downlink, the small base station 200reduces the value of proximity between the femto user equipment 340 andthe macro user equipment 320 in step 740. In this step, the small basestation 200 calculates the reduced value of proximity using Equation 2.I(A,B,n+1)=b*I(A,B,n),0≦b≦1  Equation 2

Here, ‘b’ refers to a weight used to reduce the value of proximity. Incase of success in receiving downlink at the femto user equipment 340,the small base station 200 determines that the macro user equipment 320and the femto user equipment 340 are not adjacent to each other, andthen reduces the value of proximity.

If it is determined in step 710 that uplink sub-frames are notsimultaneously allocated to both the macro user equipment 320 and thefemto user equipment 340, the small base station 200 proceeds to step750 and reduces the value of proximity between the femto user equipment340 and the macro user equipment 320. In this step, the small basestation 200 calculates the reduced value of proximity using Equation 3.I(A,B,n+1)=c*I(A,B,n),0≦c≦1  Equation 3

Here, ‘c’ refers to a forgetting factor regarding the proximity value inprocess of time.

Depending on the updated proximity value calculated through Equation 1,2, or 3, the small base station 200 can determine whether the macro userequipment 320 and the femto user equipment 340 are adjacent to eachother.

Returning to FIG. 6, after updating the proximity value in step 660, thesmall base station 200 determines whether the updated proximity value isgreater than a given threshold value in step 670. If it is determined instep 670 that the updated proximity value is greater than the giventhreshold value, the small base station 200 proceeds to step 680 andallocates downlink sub-frames to the femto user equipment 340 whileavoiding uplink sub-frames allocated to the interference candidate userequipment. If it is determined in step 670 that the updated proximityvalue is not greater than the given threshold value, the small basestation 200 returns to the aforesaid step 640.

As fully discussed hereinbefore, the small base station 200 receivesdownlink signals of the macro base station 100 and then checks sub-frametime allocated to uplink of the macro user equipment 320 located nearthe femto cell. By avoiding the checked sub-frame time, the small basestation 200 can allocate downlink sub-frames to the femto user equipment340.

Uplink of the macro user equipment 320 causing interference transmitsallocation information within the sub-frame time (for example, 4 framesin case of Long Term Evolution (LTE)). Therefore, the small base station200 can detect in advance uplink sub-frame of the macro user equipment320. Depending on the type of the sub-frame of the macro user equipment320, the small base station 200 can allocate downlink sub-frames to thefemto user equipment 340 so that such sub-frames do not overlap withuplink sub-frames of the macro user equipment 320. This method canprevent interference occurring between the macro user equipment 320 andthe femto user equipment 340.

Accordingly, exemplary embodiments of the present invention mitigateinterference which may occur between the macro user equipment in themacro cell and the femto user equipment in the femto cell. Namely, byavoidance allocation between sub-frame for macro user equipment andsub-frame for femto user equipment, it is possible to mitigateinterference.

While the invention has been shown and described with reference tocertain exemplary embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims and their equivalents.

What is claimed is:
 1. A method for mitigating interference at a smallbase station in a hierarchical cell structure, the method comprising:synchronizing a time with a macro base station using downlink signals ofthe macro base station; receiving control information from the macrobase station; identifying a sub-frame allocated for a macro userequipment based on the received control information; and allocating asub-frame to a femto user equipment located in a femto cell controlledby the small base station so as to avoid the sub-frame allocated to themacro user equipment.
 2. The method of claim 1, wherein the allocatingof the sub-frame includes: initializing a value of proximity between thefemto user equipment and the macro user equipment included in aninterference candidate user equipment list; updating the value ofproximity based on success or failure in receiving downlink at the femtouser equipment; and allocating a downlink sub-frame to the femto userequipment so as to avoid an uplink sub-frame allocated to the macro userequipment if the updated proximity value is greater than a giventhreshold value.
 3. The method of claim 2, wherein the updating of theproximity value includes: determining whether uplink sub-frames aresimultaneously allocated to both the macro user equipment and the femtouser equipment; if the uplink sub-frames are not simultaneouslyallocated, reducing the proximity value; and if the uplink sub-framesare simultaneously allocated, reducing or raising the proximity valuebased on whether the femto user equipment receives the downlink.
 4. Themethod of claim 1, wherein the control information is received through awired backhaul.
 5. A small base station for mitigating interference in ahierarchical cell structure, the small base station comprising: atransceiver for transmitting and receiving a signal; and a controllerfor synchronizing a time with a macro base station using downlinksignals of the macro base station, for receiving control informationfrom the macro base station, for identifying a sub-frame allocated for amacro user equipment based on the received control information, and forallocating a sub-frame to a femto user equipment located in a femto cellcontrolled by the small base station so as to avoid the sub-frameallocated to the macro user equipment.
 6. The small base station ofclaim 5, wherein the controller initializes a value of proximity betweenthe femto user equipment and the macro user equipment included in aninterference candidate user equipment list, updates the value ofproximity based on success or failure in receiving downlink at the femtouser equipment, and allocates a downlink sub-frame to the femto userequipment so as to avoid an uplink sub-frame allocated to the macro userequipment if the updated proximity value is greater than a giventhreshold value.
 7. The small base station of claim 6, wherein thecontroller determines whether uplink sub-frames are simultaneouslyallocated to both the macro user equipment and the femto user equipment,reduces the proximity value if the uplink sub-frames are notsimultaneously allocated, and reduces or raises the proximity valuebased on whether the femto user equipment receives the downlink if theuplink sub-frames are simultaneously allocated.
 8. The small basestation of claim 5, wherein the control information is received througha wired backhaul.
 9. A method for mitigating interference at femto userequipment in a hierarchical cell structure, the method comprising:synchronizing a time with a small base station using downlink signals ofthe small base station; receiving control information from the smallbase station; and transmitting a data to the small base stationaccording to the control information, wherein the control informationcomprises a sub-frame allocation information allocated to the femto userequipment located in the femto cell controlled by the small base stationso as to avoid a sub-frame allocated to a macro user equipment.
 10. Themethod of claim 9, wherein the sub-frame allocation information isgenerated by the small base station initializing a value of proximitybetween the femto user equipment and the macro user equipment includedin the interference candidate user equipment list, updating the value ofproximity based on success or failure in receiving downlink at the femtouser equipment, and allocating a downlink sub-frame to the femto userequipment so as to avoid an uplink sub-frame allocated to the macro userequipment if the updated proximity value is greater than a giventhreshold value.
 11. The method of claim 10, wherein the value ofproximity is updated by the small base station determining whetheruplink sub-frames are simultaneously allocated to both the macro userequipment and the femto user equipment, reducing the proximity value ifthe uplink sub-frames are not simultaneously allocated, and reducing orraising the proximity value based on whether the femto user equipmentreceives the downlink if the uplink sub-frames are simultaneouslyallocated.
 12. A femto user equipment for mitigating interference in ahierarchical cell structure, the femto user equipment comprising: atransceiver for transmitting and receiving a signal; and a controllerfor synchronizing a time with a small base station using downlinksignals of the small base station, for receiving control informationfrom the small base station, and for transmitting a data to the smallbase station according to the control information, wherein the controlinformation comprises a sub-frame allocation information allocated tothe femto user equipment located in the femto cell controlled by thesmall base station so as to avoid a sub-frame allocated to a macro userequipment.
 13. The femto user equipment of claim 12, wherein thesub-frame allocation information is generated by the small base stationinitializing a value of proximity between the femto user equipment andthe macro user equipment included in an interference candidate userequipment list, updating the value of proximity based on success orfailure in receiving downlink at the femto user equipment, andallocating a downlink sub-frame to the femto user equipment so as toavoid an uplink sub-frame allocated to the macro user equipment if theupdated proximity value is greater than a given threshold value.
 14. Thefemto user equipment of claim 13, wherein the value of proximity isupdated by the small base station determining whether uplink sub-framesare simultaneously allocated to both the macro user equipment and thefemto user equipment, reducing the proximity value if the uplinksub-frames are not simultaneously allocated, and reducing or raising theproximity value based on whether the femto user equipment receives thedownlink if the uplink sub-frames are simultaneously allocated.